Exam Study Guide 3

Convert wavelength values in nanometers to corresponding photon energy in kJ/mole.

(6.626x10^-34J/s x 3.00x10^8m/s)/(nm x 1m/10^9nm)x(6.022x10^23)x(1kJ/1000J

Calculate the amount of sodium oxalate required to react with 15mL of 0.02M potassium permanganate.

0.015L(0.02M KMnO4/ 1L)(5 mol C2O42-/ 2 mol MnO4-) = 0.00075 mol

Write the balanced redox equations for the various reactions of ferric ammonium sulfate with hydroxylammonium ion.

5Fe2+ + 8H+ + MnO4- = Mn2+ + 4H2O + 5Fe3+

What is a calorimeter

A calorimeter is an apparatus used to measure the amount of heat or energy involved from a reaction.

What is an emission (or line) spectrum?

An emission spectrum are the wavelengths of energy emitted as an atom goes from an excited state back down to a lower energy level.

Why do atoms produce an emission spectrum instead of producing a continuous rainbow-like spectrum?

Atoms produce discontinuous emission spectrum lines because their electrons can only attain a certain energy level.

Sources of error

Bubbles in titration tube - value read would be too high as the bubble would consume volume, but not contribute to the reaction. 2. Tube not primed - water dilutes the permanganate calling for more solution than actually needed. 3. The wrong solution is used or there is too much Fe2+, from not reacting properly with the hydroxylamine. More Fe2+ will require more permanganate. Too much hydroxylamine (from improper concentration) would reduce the Fe2+ requiring very little permanganate to reach equilibrium. 4. Wrong molarity 5. Half drops dont react with anything, we only accounted for the moles 6. Going past endpoint - more moles of iron reacted than was really there 7. Mass wet oxalate, weigh more, more moles of oxalate than already present 5.

Are any two emission spectra the same for different elements?

Each electron transition will emit different amounts of energy proving that no two elements can have the same spectra. Different elements have different spacings of energy levels.

Know the answers to any of the "further questions" at the end of this lab.

Fe2+ ions react with MnO4- to oxidize to Fe3+. However, this only occurs from the few Fe2+ ions from FeNH4(SO4)2's reaction

Which iron is being titrated with permanganate? (Fe 2+ or Fe3+

Fe2+ was being titrate

Describe the procedure in this lab and be able to answer why certain steps were taken and why the order mattered.

Fill flask with compressed air and mass. ii. Run CO2 generation and mass. iii. Fill flask with water and record temperature and mass. Record pressure as well. iv. Use water density at given temperature to determine volume of flask. v. Multiply the volume by the density of dry air to obtain the mass of air. vi. Subtract the mass of air from the mass of air+flask to obtain mass of empty flask vii. Subtract the mass of the flask+CO2 and empty flask to get mass of CO2 viii. Use equation to determine molar mass of the CO2 gas: Molar mass = (mass x R x K)/(P x V) ix. We determined the volume of the flask lastly, to avoid moisture in the flask.

Discuss possible sources of error that would lead to a higher or lower value calculated.

Fingerprints on the glass would contribute to a higher mass of CO2. ii. Not running the CO2 generation long enough or neglecting the second cycle will leave us with some air in the flask and a lower mass of CO2. iii. A larger than normal mass of the empty flask caused by a lower mass of air from incorrectly reading the temp and pressure used to determine the density of air will result in a lower mass of CO2. iv. Denser water means lower volume of flask. v. A higher mass of the flask means lower mass of gasses. A higher mass of the flask could be caused by moisture present when measuring the flask filled with air. vi. DI water is necessary as there are no solutes to contribute to the density of the water at a given temperature. Higher mass higher volume mass of air is larger. Mass of flask decrease. CO2 mass is higher vii. If the cork was damp, extra moisture would be added. The extra moisture would contribute to a higher mass of the gases collected. This would increase the molar mass. 1. From the start mass of air is high. CO2 mass wil be lower than air

dentify all the glassware used in this lab.

Glassware i. Rubber tubes. ii. Thistle tubes - for feeding HCl iii. Drying tube. iv. Glass tubes - 1 angled and 1 straigh

Why did it not matter if the accuracy of the thermometer was off by a few degrees?

I'm not sure but: it doesn't matter because that same error is going to be the same in the lab if used the same one.

Why do we have to calculate the heat capacity of the calorimeter?

It is important to calculate the heat capacity of the calorimeter because the calorimeter can absorb some of the heat produced from a chemical reaction. We need the heat capacity of the calorimeter to get accurate results

Explain why it was important to stir the solution and record the temperature when the first crystals began to appear in the bulk.

It is important to stir so the solution gets even temperature, and we want to take the temp when the crystals are shown because that's the freezing poin

Describe possible sources of error in this lab and how they would affect the values calculate

Lead, having a low heat capacity, takes a longtime to stabilize. Measuring from the bottom of the calorimeter where the hot metal was poured in will result in a higher final temperature resulting in a higher change in temperature of cool water, meaning there is a higher energy transfer. Meanwhile the change in the temperature of hot water is decreased. A higher change of heat of the metal divided by a lowered change in hot water gives a higher heat capacity.

Calculate the value of the Rydberg constant, in kJ/mol, for a wavelength. The Rydberg equation would be given.

RH = change in -kj/mol / n2^-2-n1^-2

Calculate the molar mass of the metal by using the law of Dulong and Petit. The law would be given. This should allow you to identify an unknown metal by its molar mass.

Step 1. Calculate the heat lost by hot water: Hhw= (mass)(heat cap of water)(change in temp of hot water) Step 2. Calculate the heat gained by cold water: Hcw= (mass)(Cap of water)(change in temp of cold water) Step 3. Calculate heat gained by the calorimeter: Hcal= (-Hhw) - (Hcw) Step 4. Calculate B (constant): Hcal/change in temp of cold water

d. What was the purpose of using the drying tube? What substance was in the drying tube

The CaCl2 helped dessicate the gas being produced. Moisture and H2O were also generated from the reaction, so the dessication tube ensured only dry CO2 gas made it to the other flask, otherwise the mass of CO2 would be erroneously high.

. What was the limiting reactant in the reaction of ferric ammonium sulfate and hydroxylammonium ion?

The Fe3+ will be reduced to Fe2+, which will be titrated

Why was it important that a stopper be used on the tube of cyclohexane?

The cyclohexane evaporates

The emission spectrum of hydrogen is a lot simpler than the emission spectrum of other elements, why?

The emission spectra of hydrogen is a lot simpler as hydrogen only has one electron making the jumps in energy level, whereas other atoms like mercury or neon have multiple electrons being energized.

Understand why we were able to say moles Fe 2+ titrated was the same as moles Fe3+ that reacted with NH3OH+ .

The first solution had iron with a 2+ and the latter had a 3+. During the reaction with the NH3OH+ (the limiting reactant) some Fe 3+ remains unreacted or unreduced. However with the first solution no reaction occurs leaving us with a higher concentration of Fe2+ than from the latter solution. The Fe2+ ions react with MnO4- to oxidize to Fe3+. However this only occurs from the few Fe2+ ions from FeNH4(SO4)2's reaction, causing us to use 10mL of KMnO4 to reach equilibrium. Because all Fe ions will be Fe2+ more KMnO4 will be required

Explain why the spectra from the lamps and room lights did not have distinct lines. Why did the compact fluorescent spectrum contain the same wavelengths as the mercury calibration lamp?

The lamps and room lights do not have distinct lines because they are incandescent bulbs, they use heated filaments instead of gaseous atoms to emit light. Electrons in in atoms do not jace a continuous range of energies, but only certain discrete values. Gas phases contribute to the distinct lines. There so many transitions its continuous, there is no distinct atomic transitions

Why can we neglect the mass of air in the flask when obtaining the mass of the water (from which we calculate the volume of the flask) but not when calculating the mass of the carbon dioxide?

The reason that we can irgrone the weight of air is because it is negligible in weight but the CO2 is not.

If a student used a metal that was wet, would the resulting measured value of the specific heat capacity be too high or too low? Explain.

The resulting measured value would be too low. The moisture adds mass to the sample and if the mass increase the energy decrease due to the relationship: Energy/(mass x temperature change).

What is the specific heat capacity of a metal?

The specific heat of a metal is the amount of energy in Joules to raise the temperature of 1 gram of metal by 1 degree.

Which gas were we collecting? How was it generated?

We are collecting CO2 generated by CaCO3 and HCl reaction.

Why did we blow compressed air in our collection Erlenmeyer? (

We blow compressed air into the flask so that there is pressure for us to account for from gaseous water in the moist air, and to obtain air without as much CO2, so that the mass of the air is not as heavy so that more mass will be retained by the empty flask measurements. Dry from washing and keep out vapo pressure.

Why was it necessary to "calibrate" your spectroscope? And, how did we do that?

We calibrated our scopes because the scope's scale does not translate to the actual wavelengths. By doing so we found the wavelengths using the scale position. We accomplished this by viewing mercury's spectrum. We were given known wavelengths and had to record one the scale where we saw the colors assigned to the wavelength. We then graphed the wavelength over the scale position to obtain an equation we will use in future measurement using the same spectroscope.

Why did we leave space around the glass tube inserted into the cork of the collection flask instead of making a tight seal with a stopper with a glass tube inserted?

We left space so that the heavier CO2 gas could displace the lighter air present in the flask, leaving only the mass of CO2.

Explain why salt was added to the ice water bath.

dding salt, it lowers the freezing point.

Why was it important to use the same thermometer for the whole experiment?

same thermometer for the whole experiment is different thermometer have different types of error